Field of the Invention
The present invention relates to a method for creating an optical binary digital transmit signal exploiting intentional mode hops and to an optical transmitter device for creating at least one optical binary digital transmit signal to be supplied to an optical transmission link for realizing this method.
Description of Related Art
In the field of optical telecommunication, optical transmission methods and respective optical transmitter devices matching the need of more and more increasing bit rates are required. Semiconductor lasers are mainly used as electro-optical conversion elements in optical transmitters. Direct modulation of a semiconductor laser induces chirp, that is dynamic wavelength changes, during modulation. This results in an undesired broadening of the optical signal spectrum and, in turn, to a reduced chromatic dispersion tolerance of a digital optical transmit signal that is created in this way. Typically, modulating a semiconductor laser, for example a DFB or DBR laser, at a modulation rate (symbol rate) of 10 Gb/s (10 GBaud) results in a dispersion tolerance corresponding to 10 km or less when using a standard single mode fiber (according to Recommendation ITU-T G.652) in the 1550 nm wavelength range.
Generally, for modulation rates of higher than approximately 4 Gb/s, externally modulated lasers are usually used instead of directly modulated lasers in order to overcome or reduce these problems. While in a semiconductor laser phase and power of the optical signal are related by a (fixed) ratio, namely the line width enhancement factor alpha, in external modulators, for example Mach-Zender modulators or electro-absorption modulators (EAM), the phase modulation can be controlled and can be reduced to zero, leaving amplitude modulation of the optical carrier only.
However, using external modulators in optical transmission systems adds complexity and cost as compared to the use of directly modulated lasers.
According to a further known method, the interaction current of a DFB laser is modulated with a small amplitude. This method is known as “chirp managed laser” and results in a relatively small extinction ratio and a frequency shift of the optical signals between mark and space bits on the order of half the modulation data rate. After the laser, a steep-edged optical filter is required, which suppresses the power in the space bits in order to increase the extinction ratio and to reduce the spectral width of the modulated optical signal.
This method, however, requires a costly optical filter and a tight control of the temperature of the laser in order to exactly match the laser wavelength with the edge of the optical filter.
The U.S. Pat. No. 5,243,608 describes an optical transmission process by wavelength shifting and a corresponding system. In this optical transmission system, intentional wavelength hopping or mode hopping is used in order to create a modulated optical binary digital transmit signal which is supplied to an optical transmission link. This modulation technique is called wavelength shift (WSK). The wavelength shift is obtained by a careful choice of the currents injected into the multiple-section semiconductor structures of the semiconductor lasers. The complete modulated signal is supplied to the optical transmission link and received, at the receiver side, by direct detection, wherein prior to the opto-electrical conversion elements an optical band-pass filter is used, in order to extract from the optical signal received the wavelength range comprising the center wavelength of a selected one of the modes between which the mode hopping occurs.
Thus, dispersion introduced by the optical transmission link, which may be a standard single-mode fiber, influences the transmission quality and thus the bit error rate of a respective transmission system.
It is therefore an object of the present invention to provide a method for creating an optical binary digital transmit signal to be supplied to an optical transmission link, especially to an optical wavelength division multiplex (WDM) transmission link, which reveals an improved dispersion tolerance. It is a further object of the present invention to provide an optical transmitter device realizing the method according to the invention.